Cage expansion in metallopentaborane chemistry: the preparation and structure of 2-{carbonylbis(triphenylphosphine)irida}-nido-hexaborane, [(IrB5H8)(CO)(PPh3)2]

Abstract

The reaction of K[B₅H₈] with [Ir(CO)Cl(PPh₃)₂] at low temperature yields [(IrB₅H₈)(CO)(PPh₃)₂] which crystallises as yellow blocks in the monoclinic space group P2₁/c with a= 10.097(2), b= 20.708(5), c= 16.493(3)Å, β= 92.85(2)°, and Z= 4. Single-crystal X-ray diffraction analysis shows that the compound has a novel structure in which the transition-metal atom has inserted into the polyhedral pentaborane cluster; the iridium and boron atoms form a pentagonal pyramid in which the metal occupies a basal position, and the structure is therefore that of a metallahexaborane. The co-ordination number of the iridium atom is seven and the geometry is a distorted, monocapped, trigonal prism which probably results from formally octahedral hybridisation about the iridium(III) atom, one of the bonds being a two-electron three-centre bond involving the apical and a basal boron atom in the metallaborane cluster. This is consistent with formal skeletal electron-counting rules.